1. Field of the Invention
The present invention relates to a group III nitride compound semiconductor laser. Especially, the present invention relates to a group III nitride compound semiconductor laser having a ridge type of a resonator.
2. Description of the Related Art
A semiconductor laser which has a cladding layer and an active layer and so on made of a group III nitride compound semiconductor (AlxGayIn1-x-yN, 0≦x≦1, 0≦y≦1 and 0≦x+y≦1) has been known. The conventional semiconductor laser is a semiconductor diode having multi-layer structure with multiple group III nitride compound semiconductor layers, which are formed on a sapphire substrate in sequence. A conventional example of the semiconductor diode, disclosed by the same applicant in Japanese Patent Laid-open No. 2000-261105, is shown in FIG. 3.
A semiconductor laser 900 shown in FIG. 3 has the following eight layers formed on a sapphire substrate 91 in sequence: a buffer layer 92; an n-layer 93; an n-cladding layer 94; an n-guide layer 95; an active layer 96 made of multiple quantum well (MQW) layer; a p-guide layer 97; a p-cladding layer 98; and a p-contact layer 99. As shown in FIG. 3, a cavity or a resonator part (a ridged resonator cavity of part) A is formed by using, e.g., photoresist and etching, and a positive electrode 901 and a negative electrode 902 are formed on the upper surface of the p-contact layer 99 and the etched surface of the n-layer 93, respectively.
The active layer 96, comprising a multiple quantum well (MQW) layer, is a semiconductor layer which functions as a main layer to oscillate laser. Each carriers (holes and electrons) injected from the positive electrode 901 and the negative electrode 902 combine in the active layer 96, that causes laser oscillation. The n-guide layer 95 and the p-guide layer 97 function to confine carriers into the active layer 96. Also, the n-cladding layer 94 and the p-cladding layer 98 function to confine laser light. And the n-layer 93 and the p-contact layer 99 are semiconductor layers which are formed in order that carriers can be injected smoothly from the negative electrode 902 and the positive electrode 901 to the layers existing between the n-cladding layer 94 and the p-cladding layer 98, respectively.
In order that the semiconductor laser made of group III nitride compound semiconductor can oscillate laser efficiently, the cross-section of electric current path of the semiconductor laser is, for example, narrowed by decreasing the contact area of electrodes, or by decreasing the width w of the positive electrode 901. In addition, the above-mentioned Japanese Patent Laid-open No. 2000-261105 suggests forming a deep ridged hole injection part B. That is, a boundary between a ridged cavity part A and the ridged hole injection part B is regarded as a boundary between the p-guide layer 97 and the p-cladding layer 98.
When forming the ridged hole injection part B, however, it is not easy for all the semiconductor lasers formed on a wafer that a boundary between the ridged resonator part A and the ridged hole injection part B functions as a boundary between the p-guide layer 97 and the p-cladding layer 98. The reason is that each one of group III nitride compound semiconductor layers formed on one wafer has different thickness according to the portion on which the layer is formed. So, as discloses in the above-mentioned official gazette, the applicant of the present invention suggests completely etching the p-cladding layer 98 even if a portion of the p-guide layer 97 is etched.
The thickness of the p-guide layer 97, however, is extremely thin, e.g., about 100 nm. So when 200 nm in thickness of p-contact layer 99 and approximately 500 nm in thickness of p-cladding layer 98 are completely etched, the p-guide layer 97 may be damaged considerably, which may deteriorate its device characteristic as a semiconductor laser.